Most insignificant article to stuff in case (5)
I believe the answer is:
least
'most insignificant' is the definition.
(I've seen this before)
'article to stuff in case' is the wordplay.
'article' becomes 'a' (English indefinite article).
'to stuff' means one lot of letters goes inside another (some letters stuff into others).
'in case' becomes 'lest' (as in 'lest we forget').
'a' going within 'lest' is 'LEAST'.
(Other definitions for least that I've seen before include "Most insignificant" , "No more petty" , "Superlative form of 'little'" , "Being lowest in importance - gone stale" , "Most slight" .)